This invention relates to a process for producing citric acid by fermentation.
Citric acid is widely used in the food and pharmaceutical industry as, for example, an acidulant in beverages or in the preparation of jams. Citric acid is also used as an antioxidant or as a stabilizer in various food products. More recently, citric acid has been employed in various detergent compositions as a detergent builder.
Hitherto citric acid has been produced from carbohydrates such as molasses by culturing moulds. Recently, it has been reported that certain strains of the genus Candida, namely Candida subtropicalis and Candida fibrae, produced citric acid from various assimilable carbon sources, such as carbohydrates, alcohols, organic acids, n-alkanes, glycerides and other fats or oils, and also crude materials containing these carbon sources, when aerobically cultured in an aqueous medium. It has also been reported that species of Candida will produce citric acid from mixtures of acetate and glucose, however, both the ratio of citric acid to isocitric acid and the yields were too low to be of commercial significance. As far as is heretofore known, no yeast has been separated which produces citric acid from acetic acid in the presence of a small amount of glucose in mineral salts media at both high yield and citric to isocitric acid ratio.
When citric acid is produced using yeast, such as from the genus Candida, usually a considerable amount of isocitric acid occurs in the culture medium as a by-product. Isocitric acid is an organic acid for which there is not a significant amount of commercial use. Thus, it is rarely desired to produce isocitric acid. Moreover, the by-production of isocitric acid makes isolation and purification of citric acid in the culture medium very complicated. Accordingly, the present invention is concerned with the suppression of by-product isocitric acid in the fermentation process while maintaining the production of citric acid in a yield equal to or greater than that of the same process in which the by-production of isocitric acid is not suppressed. Moreover, the present invention is concerned with increasing the yield of citric acid over previously known processes in such a manner that a commercially feasible process is obtained which is competitive with other sources of assimilable carbon.